1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of the Related Art
A spark plug for an internal combustion engine is attached to an internal combustion engine and used for igniting an air-fuel mixture in a combustion chamber. In general, the spark plug includes: an insulator having an axial hole; a center electrode inserted in the axial hole; a metal shell provided on an outer periphery of the insulator; and a ground electrode attached to a leading end surface of the metal shell. A spark discharge gap is defined between the ground electrode and a center electrode.
A noble metal tip containing a noble metal alloy such as platinum alloy is joined to a leading end portion of the ground electrode containing metal having heat-resistant and corrosion-resistant properties; such as a nickel alloy. The noble metal tip can improve spark wear resistance and ignitability. A technique for joining the noble metal tip to the ground electrode has been proposed, in which welding along an outer surface of a boundary between the ground electrode and the noble metal tip is carried out by means of a laser beam (for example, see JP-A-2002-313524 and JP-B-3460087).
Recently, an engine with a high compression ratio has been developed so as to increase engine output. In the combustion chamber of such an engine, the noble metal tip and the ground electrode are exposed to high temperatures. In addition, the heat dissipation property of the ground electrode deteriorates toward the distal end thereof, and the temperature of the ground electrode tends to become high at a portion closer to the distal end thereof. For these reasons, deformation due to a repetition of a cold-hot cycle may occur at a boundary between the noble metal tip and the ground electrode. This may cause an oxide scale, cracking, and the like at the boundary between the noble metal tip and the ground electrode, such that the noble metal tip may exfoliate from the ground electrode.
Further, the size of the spark plug has been reduced in response to a request for engine miniaturization, and the metal shell itself has become smaller in diameter and thickness. The size of the ground electrode provided at the leading end of the metal shell has to be reduced because the area joined to the metal shell is reduced. Consequently, the heat dissipation property of the ground electrode may be further lowered, and the foregoing problems may become more pronounced.